At first, I also thought about concrete, but then I turned to the guys from
https://www.siteprep.com/. They advised me to make a gravel pad with drainage. Clay is a capricious soil, so drainage is a must. They removed the top layer of soil, leveled it, made drainage and laid crushed stone - quickly and efficiently. For the second year, the shed has been standing level, without distortions and problems. If the budget is limited, gravel is a great option, the main thing is that the installation is done correctly.